FIG. 7 is a simplified diagram showing an exemplary conventional H-pattern contact solar cell 40 that converts sunlight into electricity by the inner photoelectric effect. Solar cell 40 is formed on a semiconductor (e.g., monocrystalline silicon) substrate 41 that is processed using known techniques to include an n-type doped upper region 41A and a p-type doped lower region 41B such that a pn-junction is formed near the center of substrate 41. Disposed on a frontside surface 42 of semiconductor substrate 41 are a series of parallel metal gridlines (fingers) 44 (shown in end view) that are electrically connected to n-type region 41A. A substantially solid conductive layer 46 is formed on a backside surface 43 of substrate 41, and is electrically connected to p-type region 41B. An antireflection coating 47 is typically formed over upper surface 42 of substrate 41. Solar cell 40 generates electricity when a photon from sunlight beams L1 pass through upper surface 42 into substrate 41 and hit a semiconductor material atom with an energy greater than the semiconductor band gap, which excites an electron (“−”) in the valence band to the conduction band, allowing the electron and an associated hole (“+”) to flow within substrate 41. The pn-junction separating n-type region 41A and p-type region 41B serves to prevent recombination of the excited electrons with the holes, thereby generating a potential difference that can be applied to a load by way of gridlines 44 and conductive layer 46, as indicated in FIG. 7.
FIGS. 8(A) and 8(B) are perspective views showing the frontside and backside contact patterns, respectively, of solar cell 40 in additional detail. As shown in FIG. 8(A), the frontside contact pattern solar cell 40 consists of a rectilinear array of parallel narrow gridlines 44 and one or more wider collection lines (bus bars) 45 that extend perpendicular to gridlines 44, both disposed on upper surface 42. Gridlines 44 collect electrons (current) from substrate 41 as described above, and bus bars 45 which gather current from gridlines 44. In a photovoltaic module, bus bars 45 become the points to which metal ribbon (not shown) is attached, typically by soldering, with the ribbon being used to electrically connect one cell to another. As shown in FIG. 8(B), the backside contact pattern solar cell 40 consists of a substantially continuous back surface field (BSF) metallization layer 46 and three spaced apart solder pad metallization structures 48 that are disposed on backside surface 43. Similar to bus bars 45 formed on upper surface 42, solder pad metallization structures 48 serve as points to which metal ribbon (not shown) is soldered, with the ribbon being used to electrically connect one cell to another.
Conventional methods for producing solar cell 40 include screen-printing and micro-extrusion. Screen-printing techniques were first used in the large scale production of solar cells, but has a drawback in that it requires physical contact with the semiconductor substrate, resulting in relatively low production yields. Micro-extrusion methods were developed more recently in order to meet the demand for low cost large-area semiconductors, and include extruding a dopant bearing material (dopant ink) onto the surface of a semiconductor substrate using a micro-extrusion printhead.
FIG. 9 is simplified diagram depicting the currently used micro-extrusion method for printing gridlines 44 onto frontside surface 42 of substrate 41 in the production of solar cell 40 (shown in FIGS. 8(A)). Substrate 41 is positioned below and moved relative to printhead 60 (e.g., in the Y-axis direction indicated by the arrow in FIG. 9) while gridline material is extruded from several nozzle outlets 69, causing the extruded gridline material to form parallel gridline structures (traces) on substrate 41. The extrusion (gridline printing) process is started when nozzle outlets 69 are positioned a predetermined distance from front edge 41F of substrate 41 such that leading edges of gridlines 44 are separated from front edge 41F by a space S. This spacing is provided in order to prevent the deposition of gridline material too close to front edge 41F, which can result in a short-circuit between gridlines 44 and conductors (not shown) that are formed on the backside surface of substrate 41. Similarly, the gridline printing process is terminated to provide a space between the lagging ends of gridlines 44 and back edge 41B of substrate 41. In comparison to screen printing techniques, the extrusion of dopant material onto substrate 41 provides superior control of the feature resolution of the doped regions, and facilitates deposition without contacting substrate 41, thereby avoiding wafer breakage (i.e., increasing production yields). Such fabrication techniques are disclosed, for example, in U.S. Patent Application No. 20080138456, which is incorporated herein by reference in its entirety.
As indicated at the lower right portion of FIG. 9, a problem with the current solar cell extrusion printing technique is that it is difficult to generate clean starts and stops to the printed traces. For example, as indicated by the rightmost traces 44A shown in FIG. 9, sputtering of the extruded material at the start of the gridline printing process can produce broken (segmented) gridlines, preventing electrical conductivity along the entire length of gridlines 44A. Similar segmented line sections are produced at the end of gridlines 44 when the gridline printing process is terminated. The broken traces occur because the flow of the conductive ink and/or support material is uneven when pumping commences (and when it stops). It is possible to avoid this segmentation of gridlines 44 by starting the pumping (and so also the printing) before substrate 41 is located below printhead 60, thereby allowing the gridline material flow stabilize before the extruded material deposits on substrate 41, and by stopping the pumping after substrate 41 has completely passed under printhead 60. However, this solution produces the problem of printing conductive ink right up to the wafer's edge, which can result in an electrical short between the upper and lower surfaces of the substrate, rendering the solar cell useless.
As indicated in FIGS. 10 and 11, another problem faced by current solar cell extrusion printing equipment is that many solar cells are formed on substrates (wafers) that are non-rectangular. That is, even if the current mechanism was improved to give clean start/stops, the current mechanism would be incapable of individually controlling the nozzles to facilitate printing on non-rectangular regions in which at least some of the gridlines are shorter than other gridlines, such as encountered when printing upon circular wafers, such as that shown in FIG. 10, or octagonal substrates, such as that shown in FIG. 11. These non-rectangular wafers require individual control over each nozzle in order to produce the desired gap (spacing S) for both the longer centrally-located gridlines and the shorter gridlines located at the edges of the substrate.
FIG. 12 illustrates yet another problem associated with current solar cell extrusion printing equipment in that, if substrate 41 is misaligned in the cross-process direction (i.e., shifted from its nominal position in the X direction) relative to printhead 60, then the resulting gridlines extruded by nozzle outlets 69 are incorrectly spaced with reference to side edges 41S1 and 41S2 of substrate 41. For example, such a shift produces a relatively large gap G1 between leftmost gridline 44B1 and side edge 41S1, and a small gap G2 between rightmost gridline 44B2 and side edge 41S2. Current solar cell extrusion printing equipment includes no mechanism for correcting such misalignment of substrate 41 and printhead 60 in the cross-process direction.
What is needed is solar cell extrusion printing equipment and an associated method for forming gridlines on a solar cell that avoids the problems mentioned above in association with the conventional gridline printing process.